! preceding function in javascript? [duplicate]

This question already has answers here: Closed 11 years ago.

Possible Duplicate:

What does the exclamation mark do before the function?

I saw a function formatted like thi开发者_高级运维s today for the first time:

!function(){}();

What is the preceding exclamation mark for? I assume it functions the same as:

(function(){})();

But... what's going on here?

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The preceding ! takes the un-parseable statement, and allows it to to be parsed by the JS engine, which in turn returns true.

function(){}();
SyntaxError: Unexpected token (

!function(){}();
>>true

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It simply makes the JavaScript parser parse it as an expression, which is necessary to execute it.

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I've tried it, it returned true. The function returns undefined, and !undefined is true.

!function(){}();
^          ^ ^
C          A  B

• A. function(){} is an empty anonymous function
• B. () executes the function (A), returning undefined
• C. ! negates undefined, which becomes true

I think they used that trick for a code golf or an obfuscated code. It is a bad practice to practially use that

Try javascript:alert(!function(){}()) in your browser address bar